Brad Kemper :
> On Dec 11, 2010, at 11:00 AM, "Dr. Olaf Hoffmann"
>
> > As already mentioned before instead of '1in is equal to 2.54cm.'
> > it should be: 'note, that 1in is equal to 2.54cm.'
> > And if the last line persists, there is no need anymore for
> > 'The absolute units consist of the physical units (in, cm, mm, pt, pc)
> > and the px unit:', this can be shortened to:
> > 'The absolute units consist of the physical units:'
> >
> > A physical unit like a centimeter is always related to a physical
> > measurement of a length, not to viewing circumstances or resolutions of
> > devices - this is the whole point about it, a centimeter is a device
> > independent absolute unit and it is defined in such a way, that this
> > entity is independent from the method used to realise/present it, else it
> > is not a centimeter.
> > There is no choice.
>
> The point you seem to be missing is that we are not trying to define inches
> and centimeters. We are defining the CSS units of 'in' and 'cm'. In some
> contexts, these have a direct relationship to inches and centimeters, and
> in others they do not. On projector screens, for instance, 'in' is just
> about as divorced in meaning from a physical "inch" as it is from the
> preposition "in".
No, as cited several times, it is written in the draft:
"
cm: centimeters
mm: millimeters
"
This means, that 'cm' represents centimeters and
'mm' represents millimeters. These are the same
letters (symbols) and words as commonly used for international
standard length units called 'centimeter' and 'millimeter'.
Therefore these are known lenghts. Indeed there
is nothing to define for the CSS WG anymore, because
this is already known and defined in other resources.
I think inch is defined in the same or a similar resource
to be dependent on meter as centimeter and millimeter
are. If the CSS 'in' differs from this inch, the CSS WG
should simply not call it 'inch' and should not relate it explicitly
to 'cm' to avoid confusion. To use 'in' as unit could still cause
confusion for those, who still use it instead of meter, but at
least a formal confusion in the draft would be avoided in
a similar way than for 'pc' (often used for the length unit parsec ;o)
If the CSS WG wants to define own units for whatever purpose,
they must not use 'cm', 'mm' and words like 'centimeter' and 'millimeter'.
However the problem with the section about the 'reference pixel'
is, that it gives the impression, that the CSS WG tries to define, what
centimeters and millimeters are, related to viewing circumstances
and device resolution. And this will typically create a contradiction
- or this can be only avoided, by fixing the viewing circumstances
and device resolution in such a way, that the result is really a centimeter
or millimeter again - but because these parameters are out of control
of the CSS WG and implementors, in major parts even out of control
for the users, such a requirement cannot be fulfilled.
And as far as I understand the discussion, it is not the intention of the
CSS WG to unify all devices or to restrict CSS to such devices, that
fulfill these requirements.
As already suggested, there could be an information, how to
present millimeters or centimeters in such problematic cases, where
the user-agent does not know anything about the resolution of the
device or for such cases, only viewing angles are relevant and not
lengths, but such information is currently not available in the draft -
or at least the wording is misleading, if it is the intention of the
reference pixel section to care about such problematic situations
and devices.
Olaf